Trigonometry is a branch of mathematics focused on the relationships between the sides and angles of triangles, with origins in Hellenistic geometry and applications in engineering. Key concepts include trigonometric ratios, the Pythagorean theorem, and functions such as sine, cosine, tangent, and cotangent, which are crucial for understanding triangle dimensions and related problems. The document also emphasizes the importance of trigonometry in both mathematical theory and practical applications.